Magnetic Circuits - Magnetic Circuit with an Air Gap
Summary
TLDRThis lecture delves into the significance of air gaps in magnetic circuits, highlighting their role in electrical machines and inductors. It explains how air gaps increase reluctance, necessitating higher magnetizing currents for equivalent magnetic field density. The lecture contrasts circuits with and without air gaps, emphasizing the substantial current increase required for those with gaps. It also discusses the fringing effect, which alters the effective cross-sectional area of the air gap, and concludes by illustrating the disproportionate magnetomotive force distribution between the core and air gap, with most energy stored in the latter.
Takeaways
- 🧲 The presence of an air gap in a magnetic circuit significantly increases the reluctance, necessitating a higher magnetizing current to achieve the same magnetic field density as a circuit without an air gap.
- 🔌 Air gaps are a natural part of some structures, such as in electrical machines where the rotor and stator are separated by an air gap, and are also intentionally designed into circuits to prevent saturation.
- ⚙️ In applications like reactors or inductors, air gaps are added to extend the range of excitation current before saturation occurs, thus protecting against loss of permeability and power.
- 📉 The addition of an air gap flattens the magnetization curve, indicating a reduced rate of change in magnetic field density with respect to the magnetizing force.
- 🔗 The fringing effect, where magnetic field lines spread out in the air gap, increases the effective cross-sectional area of the air gap compared to the magnetic material.
- 🔄 To mitigate the fringing effect, air gaps in practice are often divided into several smaller gaps.
- 📊 The magnetomotive force (MMF) required for the air gap is much higher compared to the magnetic core, even with a smaller physical length, indicating most of the energy is stored in the air gap.
- 🔋 Air gaps are crucial for applications needing high current or energy storage without the risk of saturation, as they allow for a controlled increase in MMF.
- ⚠️ Abnormal operating conditions can lead to saturation and damage in magnetic circuits, and air gaps can serve as a protective measure against such conditions.
- 🔬 The script provides a practical example comparing the excitation current required for magnetic circuits with and without air gaps, highlighting the substantial increase in current needed with an air gap.
Q & A
What is the primary impact of an air gap in a magnetic circuit?
-The primary impact of an air gap in a magnetic circuit is to increase the reluctance of the circuit, which requires a higher magnetizing current to achieve the same magnetic field density compared to a circuit without an air gap.
Why is an air gap typically included in the design of a magnetic circuit?
-An air gap is often included in the design of a magnetic circuit to prevent saturation, which can lead to a loss of permeability, an increase in current, and a loss of power. It also extends the range of excitation current before saturation occurs.
How does the presence of an air gap affect the magnetization curve of a magnetic circuit?
-The presence of an air gap causes the magnetization curve to have a less steep slope compared to one without an air gap, indicating that a higher magnetizing current is required for the same magnetic field density.
What is the practical implication of the fringing effect in magnetic circuits?
-The fringing effect causes the magnetic field lines to spread out in the air gap, effectively increasing the cross-sectional area of the air gap. This results in a decrease in the magnetic field density in the air gap compared to the core material.
Why might an air gap be divided into several small gaps in a magnetic circuit?
-An air gap might be divided into several small gaps to reduce the fringing effect, which can cause an increase in the effective cross-sectional area of the air gap and affect the overall magnetic field distribution.
How does the magnetomotive force (MMF) distribute between the air gap and the magnetic core in a circuit with an air gap?
-In a circuit with an air gap, most of the magnetomotive force is used at the air gap, even if the air gap is much smaller in length compared to the core. This means that most of the energy is stored in the air gap.
What is the role of an air gap in protecting magnetic circuits from abnormal conditions?
-An air gap can protect magnetic circuits from abnormal conditions by preventing saturation, which can be caused by high currents or energy storage, and by mitigating the effects of abnormal operating conditions that could damage the circuit.
What is the relationship between the cross-sectional areas of the magnetic material and the air gap in a magnetic circuit?
-In an idealized magnetic circuit without considering fringing effects, the cross-sectional areas of the magnetic field densities of the magnetic material and the air gap are assumed to be the same. However, in practice, due to fringing, the effective cross-sectional area of the air gap is greater than that of the magnetic material.
How does the length of the air gap affect the required magnetizing current in a magnetic circuit?
-The length of the air gap directly affects the required magnetizing current; a longer air gap increases the reluctance of the circuit, thus requiring a higher magnetizing current to achieve the same magnetic field density.
What is the significance of the difference in excitation current between a magnetic circuit with an air gap and one without?
-The difference in excitation current between a magnetic circuit with an air gap and one without highlights the increased energy requirement to maintain the same magnetic field density in the presence of an air gap, emphasizing the impact of air gaps on circuit design and operation.
Outlines
🧲 Impact of Air Gap in Magnetic Circuits
This paragraph discusses the role and impact of air gaps in magnetic circuits. The lecturer explains that air gaps can be inherent in the structure of some applications like electrical machines, where the rotor is separated from the stator by an air gap. In other applications, such as reactors or inductors, air gaps are intentionally introduced to prevent saturation, which can lead to loss of permeability and power. The air gap increases the reluctance of the circuit, requiring a higher magnetizing current to achieve the same magnetic field density. The concept is illustrated with a comparison of magnetization curves for circuits with and without air gaps. The lecturer also touches on the protective role of air gaps against abnormal conditions that could lead to saturation and damage.
🌀 Fringing Effect and Magnetizing Current
The second paragraph delves into the concept of fringing, which is the spreading out of magnetic field lines as they pass through an air gap, increasing the effective cross-sectional area of the gap. The lecturer notes that this effect can be neglected if the air gap is very small. The paragraph continues with an example comparing two magnetic circuits: one without an air gap and one with a small air gap. The example demonstrates that the circuit with an air gap requires a significantly higher excitation current to achieve the same magnetic field density as the one without an air gap. The lecturer concludes by emphasizing that even a small air gap can concentrate a large portion of the magnetomotive force, indicating that most of the energy is stored in the air gap.
Mindmap
Keywords
💡Magnetic Circuit
💡Air Gap
💡Magnetomotive Force (MMF)
💡Reluctance
💡Saturation
💡Magnetic Field Density (B)
💡Magnetic Field Intensity (H)
💡Fringing
💡Magnetization Curves
💡Exciting Current
💡Abnormal Operating Conditions
Highlights
The air gap in a magnetic circuit can either be part of the natural structure or added intentionally to avoid magnetic saturation.
In electrical machines, an air gap naturally exists between the rotating rotor and stationary parts to maintain magnetic field flow.
The air gap requires significantly higher magnetomotive force (MMF) compared to the magnetic core material to maintain the same magnetic field.
For reactors or inductors, an air gap is added to prevent saturation, ensuring better performance and efficiency in high-energy conditions.
The air gap increases the circuit's reluctance, allowing higher excitation current before saturation.
Magnetization curves show that circuits with an air gap have less slope, indicating higher magnetizing current is needed for the same magnetic field density.
Adding an air gap helps in handling high currents or storing energy in the magnetic circuit without risking saturation.
Air gaps can be implemented to protect magnetic circuits from abnormal operating conditions that could lead to saturation.
The magnetic field in an air gap spreads out due to the fringing effect, increasing the effective cross-sectional area of the air gap.
When the air gap is small, the fringing effect can be neglected, allowing the cross-sectional area of the air gap to be assumed equal to that of the magnetic core.
Multiple small air gaps can be used to reduce the fringing effect in practical designs.
Magnetic circuits with air gaps require significantly higher current to achieve the same magnetic field density, such as 5.1 amperes with an air gap compared to 0.328 amperes without.
The magnetomotive force (MMF) in a magnetic circuit with an air gap is primarily concentrated in the air gap itself.
Even a small air gap length (0.4 cm) requires much higher MMF compared to the magnetic core, showing that most of the energy is stored in the air gap.
Magnetic circuits are highly sensitive to the presence of an air gap, significantly impacting energy storage, magnetizing current, and circuit efficiency.
Transcripts
welcome back to the energy conversion
lectures
in this lecture
i will give some details about the
impact of an air gap in a magnetic
circuit
in some applications
the air gap can be part of the nature
structure of the magnetic circuit
in other types of applications
the air gap could be added to the
magnetic circuit as part of its design
let's give some examples to these two
cases
the first type of applications such as
electrical machines
the rotating part which called rotor
is physically isolated from the
stationary part by an air gap
practically the same magnetic field
lines flowing through the magnetic
material and air gap
to maintain same magnetic field lines
the air gap will require much more
magnetomotive force comparing with the
core or the magnetic materials
the second type of applications
such as reactor or inductor
the air gap is mostly added to avoid the
risk of saturation
that cause loss of permeability
increase of current and loss of power in
the magnetic circuit
implementing an air gap in a magnetic
circuit
adding additional reluctance to the
magnetic circuit
this will extend the range of the
excitation current before magnetic
saturation
occurs
to elaborate on this concept
let's look at this figure that shows the
magnetization curves of two magnetic
circuits
one with air gap and other without air
gap
it is very clear
that the curve with an air gap
has less slope comparing with the one
without an air gap
the air gap causes an increase of the
reluctance
that means a higher magnetizing current
i
is required to obtain same magnetic
field density b
comparing with the non air gap
magnetization curve
basically
if you have a magnetic circuit and you
need to apply high current or store high
energy
without the risk of saturation
you need to add an air gap to your
design of the magnetic circuit
it should be noted here that abnormal
operating condition can also lead to the
saturation and cause damage to the
magnetic circuits
therefore air gap can also be
implemented to protect the magnetic
circuits from the saturation and
abnormal conditions
to understand how the magnetic field
behaves when there is an air gap in a
magnetic circuit
let's assume we have the following
magnetic circuit that consists of a
magnetic core and an air gap
in the previous lectures we assume that
the magnetic field lines
are confined and uniformly distributed
within the magnetic materials
and air gap
therefore
the cross-section areas of the magnetic
field densities
of the magnetic material and the air gap
are same
in other words
ac equal to ag
and bc equal to bg
if you remember
because of this assumptions we were able
to represent the magnetic field lines of
the magnetic circuit by the mean
magnetic field path phi
any practice
the magnetic field lines phi
that flow
through the air gap is spread out as
shown
this behavior
known as fringing of the magnetic field
lines
the flinging behavior
cause an increase in the effective
cross-section area of the air gap
in other words
the cross-section area of the air gap
will be greater than the cross-section
area of the magnetic material
or
a g greater than ac
and therefore bg less than bc
it should be noted here that if the air
gap is very small
the fringing effect can be neglected
therefore
we can assume that a g equal to ac
and therefore
bg equal to bc and equal to phi over ac
any practice
if we need to add an air gap to a
magnetic circuit
the air gap will be divided into several
small air gaps
to reduce the fringing effect
now let's assume we have these two
magnetic circuits
and try to determine
the excitation or magnetizing current i
for both circuits
as you can see
circuit a
is implemented without air gap and
circuit b
is implemented with
a small air gap
since the magnetic field density b
is known and equal to 0.6 tesla
the magnetic field intensity sc
for the soft cast steel core
can be determined for both circuits from
the bh curve
and it is equal to
325 ampere turns over meter
let's start with the circuit that has no
air gap
the excitation current can be determined
as follows and it is equal to 0.328
amperes
using the same procedure the excitation
current
for the magnetic circuit with air gap
can be determined as follows
and it is equal to
5.1 amperes
actually i provided this example
to highlight the following two points
the first point is that the magnetic
circuit with the air gap will require
higher current of
5.1 amperes
to achieve magnetic density of 0.6 tesla
however
the magnetic circuit without air gap
will require only 0.328
amps
to achieve magnetic density of 0.6 tesla
these results confirm what i have
explained at the beginning
of this lecture
the second point is that the
magnetomotive force of the air gap of
circuit b
is equal to 1909.8
ampere turns
while the magnetomotive force of the
magnetic
core
of the same circuit is equal to 130
ampere turns
that means
even with a small air gap of 0.4
centimeter length compared with 40
centimeter core length
most of the mmf
or magneto motor force is used at the
air gap
in other words
most of the energy
is stored in the air gap
now let's conclude this lecture at this
point and will continue in the next
lecture
thanks for your attention i am essan and
nebby and it was a pleasure sharing this
lecture with you thank you
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